Hitherto, the present inventors have developed and disclosed a method of coloring a black magnetic powder by coating the black magnetic powder with an interference film comprising silica/titania (hereinafter referred to also as titanium oxide) (e.g., JP-A-10-330644).
The inventors have further developed and disclosed methods for coating base particles, e.g., an iron powder, with a titania film, which include: a method of coating with a titania (hereinafter referred to also as titanium oxide) film in a liquid phase by the hydrolysis reaction of a metal alkoxide (e.g., JP-A-6-228604); and a method of coating with a titanium oxide film in a liquid phase by the neutralizing hydrolysis reaction of a metal salt using a titanyl sulfate solution (e.g., JP-A-2000-345072). Besides these, a method of coating with a titanium oxide film in a liquid phase by the thermal hydrolysis reaction of a metal salt using a titanium(IV) chloride solution (e.g., JP-A-5-286738) has been disclosed.
Incidentally, the methods of coating with a titanium oxide film or the like which have been developed by the present inventors have various problems.
In the case where a metal alkoxide is used as a feed material for a titanium oxide coating film, there have been problems, for example, that since the hydrolysis reaction of the metal alkoxide proceeds exceedingly rapidly, the reaction system should be kept at a constant temperature and a constant humidity, that the metal alkoxide used as a feed material is expensive, and that it is necessary to use as a reaction solvent an organic solvent which is a hazardous substance, e.g., an alcohol, and the cost of explosion-proof equipment or the like is high.
On the other hand, the method of coating with a titanium oxide film in a liquid phase by the neutralizing hydrolysis reaction of a metal salt has a drawback that the base particle suspension becomes acidic in an initial stage or final state of the reaction. Furthermore, the method of coating with a titanium oxide film in a liquid phase by the thermal hydrolysis reaction of a metal salt has a drawback that the base particle suspension becomes acidic at high temperatures.
In the method of coating with a titanium oxide film by the neutralizing hydrolysis reaction of a metal salt, it has been difficult to use an iron powder, which is highly corrodible in acid environments, or the like as the base.